The 386SL adds a cache controller, memory controller, AT bus controller, and EMS 4.0 hardware, which is why it has more than three times the transistors. I checked multiple independent sources before posting, and they all said 855,000 transistors for the 386SL. (Sources from 1990s, so they're not parroting Wikipedia.) The cache controller has a (relatively) huge amount of tag RAM, which accounts for a lot of the additional transistors.

Afaik 386SL was in fact not a 386, but a cutdown 486. This might explain the difference in transistor count.

No, if you look at the die, the 386SL is nothing like the 486. The 386SL more-or-less has a 386 inside it, taking up about 1/4 of the chip. See this floorplan diagram: http://www.bitsavers.org/components/intel/80386/240814-005_3...